If you can't decide if you prefer meteor impacts, volcanoes or some other explanation for Earth's biggest mass extinction events, take heart: you no longer have to choose.

A new statistical study of mass extinctions throughout the history of life on Earth is backing the idea that no single meteor, volcanic eruption or other lone gunman is ever to blame.

This even applies to the Cretaceous-Tertiary event that brought the end of dinosaurs 65 million years ago, the US researchers say.

Instead, the worst die-offs happen when some sort of interminable, multi-generational pressure on life is combined with a few powerful blows.

It's what is now being called the press-pulse theory of mass extinctions.

Reading the Cretaceous-Tertiary extinction literature and conversations with colleagues "made me wonder whether the simplistic scenario of 'everything's fine until one day in June when the asteroid hits and everything goes to hell-in-a-hand-basket' really explains the diversity of data," says plant fossil expert Associate Professor Nan Arens of Hobart and William Smith Colleges in New York.

Wouldn't it make more sense, she surmised, if certain species were already vulnerable when the triggering event happened?

To test the idea, she and then-undergraduate student Ian West compiled a large database of marine organisms and their extinctions through geological time.

They divvied up the past 488 million years into four groups: suspected meteor impacts (pulses), gigantic volcanic flood basalt eruptions (presses), periods with neither presses nor pulses, and times when press and pulse coincided.

They then compared average extinction rates in each group.

Flood basalt eruptions are considered presses because they release vast amounts of greenhouse gases into the atmosphere and can change the Earth's climate.

Single or multiple event?

The researchers found similar extinction rates when a pulse or press occurred by itself, and when neither was occurring, says Arens.

"However, when an impact occurred during a time of volcanic flood, that produced higher extinction rates."

"The goal of our work was to come up with a unifying theory of mass extinctions," says West.

They also wanted to make the theory applicable to the rapid extinctions now being seen as a result of accelerating climate change (press) and the ongoing destruction of wild habitats by human activities worldwide (pulse).

"[The theory] is essentially a more eloquent way of saying what I and many other palaeontologists have been saying for many years," says Professor Gerta Keller of Princeton University.

"Namely that the impact-kill hypothesis is all wrong. Impacts alone could not have been the killing mechanism for the K-T [Cretaceous-Tertiary event] or any of the other major mass extinctions."

Volcanic activity

In the late Cretaceous case massive volcanism, the Deccan Traps eruption in India, and attendant climate change, coincided with an impact that pushed highly stressed biota over the brink.

"I'm very happy they have done the analysis based on the literature and come up with the same conclusions that palaeontologists have been preaching all along," Keller says.